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Thu Feb 1 20:17:00 EST 2018 · https://mail.python.org/pipermail/python-dev/2018-February/152048.html

On 2/1/2018 7:34 PM, Elvis Pranskevichus wrote:
> There appears to be a critical omission from the current dataclass
> implementation: it does not make hash=True fields immutable.
> 
> Per Python spec:
> 
> "the implementation of hashable collections requires that a key’s hash
> value is immutable (if the object’s hash value changes, it will be in
> the wrong hash bucket)"
> 
> Yet:
> 
>      import dataclasses
> 
>      @dataclasses.dataclass(hash=True)
>      class A:
>          foo: int = dataclasses.field(hash=True, compare=True)
> 
>      a = A(foo=1)
> 
>      s = set()
>      s.add(a)   # s == {a}
>      a.foo = 2
> 
>      print(a in s)
>      print({a} == s}
>      print(s == s)
> 
>      # prints False False True
> 
> 
> This looks to me like a clearly wrong behavior.
> 
> 
>                                      Elvis

Data classes do not protect you from doing the wrong thing. This is the 
same as writing:

class A:
     def __init__(self, foo):
         self.foo = foo
     def __hash__(self):
         return hash((self.foo,))

You're allowed to override the parameters to dataclasses.dataclass for 
cases where you know what you're doing. Consenting adults, and all.

Eric.

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[Python-Dev] Dataclasses and correct hashability